life of a metal arc lamp

What is the longest you could expect a mercury or metal halide lamp to last? A person I know showed me a lamp in his lot that he says has been on since he took over the property in 1996. The photo switch is not working so it never shuts off. It looks rather dim but it's on. And he says two other parking lot lights with working photo cells are burned out.

Just a comment, but have you ever had a lightbulb die except when you first turn it on? I suppose if you had a big surge it might go while on, but almost 100% of the failures occur when the initial surge is applied. this applies more to incandescent bulbs rather than arc types, but they often die then, too.